To place a satellite in a required orbit, the location and attitude of the satellite in relation to the earth must be known. The attitude of the satellite is normally determined through use of sensors on board the satellite. Information from the on-board sensors may either be processed with on-board computers, or sent by telemetry to a ground station for processing to determine the orientation of the satellite in inertial space.
Generally, a satellite includes an earth chord sensor, which is an infrared sensitive device that detects the heat of the earth as the satellite spins and sweeps the sensor field of view across the earth to measure its chord. The earth chord sensor typically generates a near trapezoidal signal. An analog circuit may be used to generate and report time stamps when the output voltage of the earth chord sensor falls below a predetermined threshold at the leading edge of the earth and when the output voltage of the earth chord sensor rises above the same predetermined threshold at the trailing edge of the earth. The difference between the two time stamps indicates the approximate time it takes the satellite to sweep through the length of the earth.
Prior art systems for processing the output of the earth chord sensor effectively calculate the earth chord time as long as: 1) the satellite is spinning at a high enough rate; 2) the earth chord is short enough for the sensor to generate a near trapezoidal signal; and, 3) there is little system noise (e.g. DC bias, low frequency and high frequency noises). In particular, if the satellite is spinning at a low rate, the earth chord sensor generates a slightly irregular signal. At low spin rates, when the earth chord sensor first encounters the earth, the signal drops to a softly rounded negative voltage peak. The true leading edge peak may be somewhere in this rounded peak. As the field of view of the sensor sweeps across the earth, the signal gently transitions upward to another softly rounded voltage peak. The true trailing edge peak may be somewhere in this peak. When the earth entirely leaves the field view of the sensor, the signal slowly moves towards a zero voltage.
The true earth chord time for satellites is the time between the true leading edge peak and the true trailing edge peak. However, with the softly rounded voltage peaks in the signal from the earth chord sensor, the true leading edge peak and the true trailing edge peak may not be locatable by the analog circuit. U.S. Pat. No. 5,922,033 provides a method for determining earth chord times for a satellite spinning at a low spin rate. However, the technique in U.S. Pat. No. 5,922,033 presupposes a particular signal characteristic and cannot be used for a wide range of satellite spin rates. Therefore, there remains a need for a system and method for determining earth chord times for a satellite spinning at a range of spin rates including a low spin rate.